a. Field of the Invention
The invention concerns an electric-motor drive for a bowden-cable window lifter. More particularly the invention is directed to an electric-motor drive for a bowden-cable window lift including a brake for stopping unintended or forced reverse actuation of the window lift by manually urging the window pane into an open position.
b. Description of the Prior Art
It is known as regards window lifters for automobiles and similar vehicles that a stop or other braking system must be provided in the mechanism chain between the drive and the window pane to be moved. Such a brake prevents the window pane from being moved down by applying force on the pane itself.
As regards manually powered bowden-cable window lifters, a helical spring acting as a braking spring is provided for that purpose, which in its function as a wrapping spring encloses both the crank bolt and a fork affixed to the cable reel and of which the bent-off ends are displaced by the crank pin when the latter is actuated. In this known construction, manual actuation in one direction contracts the spring, however, upon actuation of the cable reel itself through the pane in an opposing direction the spring undergoes spreading (expansion) and presses itself against the wall of a surrounding braking cup.
Heretofore such a braking spring has been omitted in bowden-cable window lifters driven by electric motors. The mechanism chain of such a conventional bowden cable drive passes from the electric motor through a worm affixed to the motor shaft to a worm wheel coupled to a cable reel for joint rotation. As a result, the force transmission--and hence also the worm motion at the engaging site on the worm wheel--is blocked nearly automatically if the mechanism chain is actuated in reverse through the window pane because as a rule the worm wheel cannot fully rotate a worm. However, this self-locking depends on the pitch angle and the flank shapes of the worm wheel toothing and the level of wear experienced by the engaging portions of the worm and worm wheel. If the engaging angle between the worm and worm wheel is relatively shallow, and if the tooth or thread flanks are sufficiently smooth, then self-locking no longer takes place, and the worm can indeed be rotated easily by the worm wheel.
Such a relatively shallow angle however is desirable for drive means of the kind described herein because only with such shallow angles is it possible to translate the window pane by comparatively few motor revolutions to close, lower or open the window. If the worm pitch were steeper, and with a matching worm wheel, substantially more motor revolutions would required to open or close the window and accordingly a substantially longer time would be required.
As a result of the desired driving speed relationship in known worm gear/worm wheel mechanisms, the self-locking ordinarily provided by the worm and the worm wheel no longer is present. Therefore it is the object of the invention to create a drive of the stated kind which is capable of stopping displacement of the pane in spite of a shallow worm pitch if the attempt is made to move the mechanism chain by means of the window pane.